1. Field of the Invention
The invention pertains generally to the art of elevator mechanisms. More specifically, the invention relates to a sectional lift truck mast of the type having nested uprights. Certain aspects of the invention are directed to a so-called "triplex" mast having three sections. One section is stationary, a second is extensible from the first, and a third from the second so that the mast can reach a greatly-extended height as compared to its collapsed height.
2. Description of the Prior Art
Industrial fork lift trucks normally have a vertically extensible cargo mast mounted on the forward end of the truck. It is generally desirable that the collapsed height of the mast be fairly low to ensure vertical clearance through passageways and access into boxcars or the like. But on the other hand, the maximum fork height should be as great as possible to permit stacking cargo to a considerable height such as in an open yard or warehouse. Thus, the triple sectional or triplex mast is often desirable since it has great height, fully extended, yet a low profile in the collapsed position.
The mast will have a load carriage for lifting the cargo. The load carriage reciprocates on the inner extensible mast section. The distance the load carriage can be raised from its fully lowered position without any extension of the mast occurring, the so-called free lift height, is another important consideration. This allows a cargo to be lifted to the uppermost position consistent with overhead clearance without extending the mast itself, for example, inside of a boxcar where elevation of the mast to increase the cargo height would be prohibited.
The typical mast has a power cylinder which cooperates with a chain lift arrangement for raising the load carriage and sequentially extending the mast sections. One type of commercially available cylinder which can be used with a triplex mast has vertically extensible cylinder rods selectively movable in opposite directions out of the cylinder. In the collapsed position of the mast and the fully lowered position of the load carriage, the cylinder rods are withdrawn and the cylinder is lowered. In raising the load carriage to its free lift height, a first cylinder rod strokes out to the extent permitted by the clearance between the top of cylinder and the mast. Successively the other cylinder rods are extended in raising the mast. In the process the cylinder is elevated. When the last cylinder rod has been fully extended, the mast reaches its maximum overall height and the cylinder has been elevated several feet, tending to obstruct the operator's vision through the mast.
Aside from creating an obstruction, another disadvantage is that the cylinder is not solidly supported on the mast frame. With the mast in a collapsed position, the lower cylinder rod projects out slightly preventing the cylinder from resting solidly on the mast frame. When the mast is elevated, the cylinder rod extends out raising the cylinder as well as the entire load of the mast. Obviously there is a degree of instability in supporting the mast on a small extended cylinder rod to say nothing of supporting a load of several thousand pounds in an elevated position.
In masts where the power cylinder remains fixed and does not elevate, usually a cylinder is required which is larger than the described elevating type. Since this large cylinder is mounted directly in front of the operator, his view to the front tends to be obstructed by it and other structure such as the hydraulic lines and the load chains. Thus, whether the power cylinder is fixed or movable vertically as with prior art constructions, operators have, at times, been forced to lean out of the side of the truck to see around an obstructing cylinder.
Many mast sections have used generally C-shaped channels for carrying and for engaging rollers. Forward bending loads of the mast are taken by the rollers traveling on the end flange portions of the channels and other rollers, to absorb side thrust loads, are provided. These thrust loads can be considerable when an unbalanced load is on the fork of the load carriage.
The space consumed by most prior thrust arrangements has limited the extent to which mast sections can be nested and, therefore, further contributed to the problems of limited visibility. An arrangement has been proposed to reduce cost and provide nesting through the use of I-beams and cocked forward load absorbing rollers. That is, the axes of the rollers are inclined at a slight non-perpendicular angle with respect to a path of mast section elevational travel. The upper roller is cocked in one direction and lower roller in the opposite direction. Since the rollers are cocked, portions of the forward bending loads are applied to the forward load absorbing rollers at all times as axial rather than radial loads. In addition, the rollers are required to take axial loads resulting from imbalanced side thrust loads. Roller bearings which are designed to withstand axial loads as well as radial loads are more expensive and have shorter lives than those which have only radial loads applied.
Moreover, in the described prior mast arrangement, the axial loading component from the forward bending loads is applied in one direction while the axial component resulting from thrust loads is taken in the opposite direction. Thus there are axial loadings in both directions where cocked rollers are used.
Normally, a system of latches is employed to control the relative movement of mast sections when the mast is extended or collapsed. For example, at some point during the elevation of the load carriage, one latch is tripped to release the first extensible mast section from the outer or stationary section. The extensible section in turn carries a latch which releases the inner section, the latter continuing to elevate until it reaches its full extension.
A variety of latching mechanisms have been proposed in the prior art, but for one reason or another none has been entirely satisfactory. Some have sacrificed free lift height. Others have employed complex latching systems which, aside from the cost, require an inordinate amount of adjustment and maintenance.
Most prior latching devices have contributed to poor bearing conditions for in a typical prior triplex mast, the inner mast section will be fully extended before the latches release the intermediate mast section permitting it to extend. Thus, the inner mast section is fully extended upwardly before the intermediate section is released to commence its upward travel. This means that the bearings between the inner mast section and the intermediate mast section are overworked much of the time since many lifts do not require extension of the intermediate section. In addition these bearings are under their worst loading conditions when the inner section is extended before the intermediate section commences to move. This is because the bearings are moved closer together which is the worst loading condition. The bearing loads are in the best condition with the bearings spaced widely apart. In other words, the loading conditions are a function of the amount of vertical overlap between mast sections. As a result, the bearings interposed between the inner and the intermediate mast sections are excessively loaded and experience excessive wear since there is relatively little vertical overlap in many of the frequently encountered lift heights reached by the extended position of the inner section alone.